Portable handheld electronic devices, such as the iPhone® multifunction device by Apple Inc., have a touch screen in their front face, where an earpiece speaker or receiver used for telephony is located. When the device is being held against the user's ear during a phone call, a proximity detector automatically senses this condition using an infrared sensor that is built into the device, and on that basis turns off the touch sensitive display screen of the device. The proximity detector can also automatically sense when the device is then moved away from the user's ear, in which case the touch screen is re-activated. This is achieved by measuring the signals of the infrared sensor's emitter and complementary detector, where the emitter transmits an infrared signal that is reflected by a nearby object (e.g. the user's head) and picked up by the detector. A stronger received signal may be interpreted by the detector to mean that the object is closer, while a weaker received signal means the object is farther away.
In many instances, the proximity sensor is combined with an ambient light sensor (ALS) which senses ambient visible light intensity. An ambient light detector process uses the sensed visible light intensity to, for example, adjust the touch screen display lighting. The ALS should have a field of view (FOV) which is larger than that of the proximity sensor, so that light from all different directions can be detected. When the proximity sensing device and the ALS device are packaged very close together, however, the field of view of the ALS may have to be limited to that of the proximity sensing device.